Page 332 - Clinical Anatomy

ECA5 7/18/06 6:51 PM Page 317

The skull 317

Development of the mandible and the teeth are considered on pages 323
and 324.

Clinical features

Fractures of the skull

Imagine the skull as a rather elastic sphere completely filled by semi-fluid
material; a violent blow on such a structure will produce a splitting effect
commencing at the site of the blow and tending to travel along lines of least
resistance. The base of the skull is more fragile than the vault, and is thus
commonly involved by such fractures. The petrous part of the temporal
bone, however, forms a firm and rarely involved buttress of the skull base,
the fracture line passing through less resistant areas, particularly the
middle cranial fossa, the pituitary fossa and the various basal foramina.
A localized severe injury, in the adult, may produce a depressed com-
minuted fracture; the infant’s skull is much more elastic and a similar injury
here will result in a ‘pond’ depressed fracture, rather like the dimple pro-
duced by squeezing on a ping-pong ball.

Localizing signs in cranial fractures

Fractures of the anterior cranial fossa may involve the frontal, ethmoidal
and sphenoidal sinuses and be accompanied by bleeding into the nose or
mouth. In such cases C.S.F. leakage from the nose implies coexisting tearing
of the meninges; the subarachnoid space is thus put in communication with
the exterior via the nasal cavity with consequent risk of meningitis.
Fractures involving the roof of the orbit are frequently associated with
blood tracking forward beneath the conjunctiva (subconjunctival haemor-
rhage); this must be differentiated from a small flame-shaped haemorrhage
of the conjunctiva caused by direct injury to it.
A ‘black eye’ is not necessarily indicative of an anterior fossa fracture; it
may be produced also by direct contusion of the soft tissues or by blood
tracking down deep to the aponeurotic layer of the scalp (see ‘The scalp’,
page 312).
Anterior basal fractures may involve the cribriform plate (with anosmia
— loss of smell— due to rupture of fibres of the olfactory bulb) or the optic
foramen (with primary optic atrophy and blindness).
Fractures of the middle fossa may produce bleeding into the mouth
(sphenoid involvement), bleeding or C.S.F. leakage from the ear, and facial
and auditory nerve injury. Aural bleeding may, of course, be produced by
direct injury to the ear—for example, rupture of the drum—without neces-
sarily implying a skull fracture. Because of its long course, the abducent
(VI) nerve may be damaged with diplopia and paralysis of the lateral rectus
muscle.
Posterior fossa fractures are occasionally accompanied by cranial nerve
involvement. These fractures are suggested clinically by bruising over the
mastoid region extending downwards over the sternocleidomastoid.